The present invention relates to an optical integrated circuit and an optical apparatus used in an optoelectronics device such as an optical communication apparatus or an optical disk recording apparatus, and in particular to an optical integrated circuit and an optical apparatus in which various aberrations caused by variation in wavelength of a light source when a semiconductor laser is used as the light source are corrected.
Conventional optical components used in fields such as an optical communication system and optical information processing comprise bulk components such as lenses, prisms and gratings mechanically assembled. Since the above described optical components have large outer dimensions, therefore, they cannot meet a demand for size reduction. Further, they are expensive. Further, because of assembly using mechanical coupling, the above described optical components lack safety for long time use and are inferior in reliability. Because of these various problems, the concept of an optical integrated circuit (optical IC) comprising a plurality of devices integrated on a single substrate has been introduced in recent years to study significant reduction in size and cost of optical components. That is to say, an optical IC comprises optical components obtained by integrating a photodetector device, a light emitting device, a lens of waveguide type (thin film type) and a grating on a single substrate.
One component of the optical IC is a grating coupler, which is a waveguide diffraction grating formed in an optical waveguide. The grating coupler functions to make an optical beam incident on the optical waveguide and make the optical beam radiate from the optical waveguide. The grating coupler is thus one of key components of the optical IC.
Concrete design methods of the above described grating coupler are discussed in J. H. Harris, et al. "Theory and Design of Periodic Couplers", Appl. Opt., 11, 10 (1972), and T. Tamir and S. T. Peng, "Analysis and Design of Grating Couplers", Appl. Phys., 14, (1977), for example. As for optical components formed into an optical IC by using a grating coupler, examples of their application to an optical head for optical disk apparatus are described in JP-A-61-85641 and JP-A-61-296540.
When a semiconductor laser is used as the light source, the above described prior art has the following problems. That is to say, the wavelength of light emitted by a semiconductor laser typically changes because of nonuniformity in operation temperature and manufacturing process for fabricating the semiconductor laser. A first problem will now be described. In case the emitted wavelength is not single, the incidence angle whereat the beam can enter the optical waveguide is changed by the grating coupler, resulting in a problem of lowering in incidence coupling efficiency. Further, there is also a problem that the radiation angle changes in the same way as the foregoing description when a grating coupler is used at the exit side.
A second problem will now be described. When a grating coupler having nonequal pitches and taking a curvilinear shape is employed as a focusing grating coupler used as an objective lens of an optical head, the spot size of a beam at a focal point must be reduced up to nearly the diffration limit in order to read out high-density information recorded on an optical disk medium. Therefore, the numerical aperture (NA) of a lens, which is defined as the diameter of lens/focal length, must be not less than 0.45. In case of the above described conventional focusing grating coupler, however, the wavelength deviation .DELTA..lambda. of the laser light must satisfy a strict value represented as .vertline..DELTA..lambda..vertline.=9.8.times.10.sup.-4 .mu.m when NA is 0.45. Further, deviation .delta. of the laser light from the optical axis and deviation .DELTA.N of the effective refractive index of the guided optical beam must respectively satisfy the following values. EQU .vertline..delta..vertline.=6.9.times.10.sup.-4 deg EQU .vertline..DELTA.N.vertline.=9.8.times.10.sup.-4
These values are extremely strict and are not at a practical level. Especially, since .delta. is small, the above described conventional focusing grating coupler cannot be applied to an integrated optical pickup in which high-speed access is implemented by propagating surface acoustic waves at right angles to the guided optical beam and deflecting the beam to the left and right with respect to the optical axis by using the acoustic waves, resulting in a drawback.
A third problem will now be described. In the above described conventional optical head fabricated as an optical IC, a beam splitter for detecting a signal supplied from the optical disk is constituted as a so-called coplanar optical device. Therefore, the reception angle (i.e., magnitude of deviation of an incidence angle of light with respect to a device which does not cause significant deterioration in device characteristics and hence is permitted) is small, and the detection range of a focal point error signal is narrow. The range in which focusing servo functions is limited to a narrow range, and characteristics deterioration caused by a change in wavelength of laser light is significant, i.e., so-called chromatic aberration is significant, resulting in a subject to be solved.
A fourth problem will now be described. When the optical disk is not in parallel to the above described conventional optical head, the optical axis obtained when the light reflected by a surface of the optical disk is introduced to the optical waveguide path again is deviated from the center axis of the beam splitter. Therefore, an offset is caused in a tracking error detection signal, resulting in a subject.